Mirror



Aug. 18,1942. y v.HA R|s ,,;2,Z9,206

' MIRROR Filed March 14, 1935 mkkog'pkersmsu PARABOL/QCOMPOSED ,ormv ALLOY COMPRISING cbaALT, 15% T0 35 CR;

' j 0.5% TQ 6%? AND/0R MQ AND ABOUT 1% c.

\NvEMoR ALBERT V. HARRIS BY E i ATTO RN EY Patented'Aug. 18, 1942 Albert V Harrls, Bloomfield, J., assignor, by,

17 mesneassignmentspto Haynes Stelm c Pally, a corporation of Indiana a )1 Application March 14,1935, Serial No. 11,044 2 CIalrns. (cuss-.105)

refers more specifically to high intensity electric arc light projectors of the types used as searchlights and fioodlights, and to rolled, optically figured, and polished metal mirror-s.

In recent years the demand forintense illumination for ships and harbors, airports, andqvarious naval, military, and industrial purposes has led to the development of electric arc lights of extremely high intensity. Such lights commonly comprise slender carbon electrodes impregnated or cored with flame-producing material, for instance, rare "earthmetal fluorides. These electrades are forced to carry heavy currents, often upwards of 200 amperes at 90 volts for a 16 millimeter diameter electrode. The intense heat developed and the hot particles of carbon and of core material thrown bythe arc, severely damage in a short time the materials ordinarily used for the light reflecting surface of the projector.

i t The invention relates to lightprojectors, and

atmosphere, of a high percentage reflectivity, 'mechanical strength, and an accurate optical figure; i q a a This and other objects of the invention areattamed by a figured and polished hot-rolled metal mirror having substantially the composition: to 35% chromium, 0.5% to 6% metal of the group consisting of tungsten and molybdenum, carbon in amounts not over about 1.25%, and the remaindercobalt with which there may be associated small amounts of iron, nickel, manga nese, and silicon. Preferably, the tungstenmolybdenum group metal forms at least 2%-of the composition. A preferred composition consists of chromium, 5% tungsten, 1% carbon,

and the remainder cobalt.

silvered glass mirrors have beenmost commonly used as the reflecting means in light projectors. Such mirrors are extremely fragile, being unableto withstand, -'for example, the vibration of more thanthree or four rounds of heavy gunfire or even slightly careless handling. Moreover, the intense heat of a high intensity arc tends to alter the optical figure of a glassmirror,

and elaborate cooling arrangements are required. Hot particles of carbon and of core material pit the glass and adhere to it. Further, humid or salt air and the corrosive products of the are rapidly attack the silver backing of these mirrors.

It has been proposed to use various bright metals, either in the form of a solid plate or aflixed invarious ways as a surface to a backing material, as the reflecting surface of a light pro- Jector. Thus, nickel, cadmium, chromium, and rhodium plated mirrors and solid stainless chromium steel mirrors have been proposed, as well as nickel alloy mirrors containing chromium and molybdenum or tungsten. None of these materials has been a pronouncedsuccess, for the rea son that none has possessed to a fully satisfactory degree the ability to retain persistently a high percentage reflectivity, mechanical strength, re sistance to staining and pitting, resistance to hot I have found that a mirror of the composition and physical condition above specified possesses a number of unique characteristics which make it preeminently suitable for efficient cooperation with thehigh intensity arc light source. It possesses a very high percentage reflectivity for the entire visible spectrum, comparing favorably in this respect with silver-backed glass; audits refiectivity is retained far longer under adverse conditions than is the reflectivity of the silvered glass. It effectively resists tarnishing by salt water, humid air, fluorides, hydrogen sulfide, cyanogen compounds, and other corrosives encountered in use. It persisently retains its optical figure under the most difiicult conditions of heat and ,mechanical shock, and its mechanical strength is such that it requires no specialprotection or cooling devices. It is extremely hard, even at elevated temperatures, and resists scratching and rough treatment. The sputter in" of high intensity are carbons which frequently throws hotparticles of carbon violently against the mirror with disastrous consequences to silvered glass or tometal structures heretofore used, has little or no effect upon the mirror of the invention: the hot carbon particles donot mar or adhere to the mirror.

Aside from the extraordinary characteristics of long life and sustained eificient service possessed by the novel mirror of the present invention, its use inlight projecting apparatus results in many advantages of simplicity, lightness, rugparticles ofcarbon and salts, and an accurate It is an object of the'invention to provide, a

type provided with a mirror characterized by a persistent retention. under the normally destrucgedness, and ease of operation in such apparatus, for the reason that the cumbersome cooling dew vices ordinarily required may be eliminated. In

fact, the mirror itself may constitute the rear 010- i sure for the light projector, thus improving the lightness, balance,

and maneuverability of the five influences of the high intensity are and projector by the elimination of the separate rear closure and cooling devices usually found necessary.

A typical embodiment of the invention is illustrated in the attached drawing which shows a searchlight barrel l3 closed at one end by a mirror l0 composed of cobalt, chromium, carbon, and either tungsten or, molybdenum or both, in the proportions described herein. A light source is supplied by suitable electrodes II and I! to which an electric current is supplied by appropriate leads (not shown) in the customary manner.

The mirror of the invention has been made and tested in sizes up to about 36 inches, and has proved to be far superior to any mirror heretofore known, particularly when used in a light projector in combination with a high intensity carbon are electric light source.

It may sometimes be desired to increase the ductility of the mirror of the invention at the sacrifice of some of its resistance to scratching and corrosion. In such a case, the content of tungsten-molybdenum group metal may be lowered below the preferred minimum of 2%, and may even be entirely eliminated. V

I claim:

1. A rolled, optically figured, and polished metal mirror characterized by. its persisent retention, when exposed to the action of corrosive media, heat, or mechanical shock, of its optical figure and reflectivity, and having substantially the composition: 15% to 35% chromium, 0.5% to 6% metal of the group consisting of tungsten and molybdenum, carbon in substantial amounts in the neighborhood of 1% but not over about 1.25%, and the remainder cobalt and incidental impurities such as small amounts of iron, nickel, manganese, and silicon.

2. A rolled, optically figured, and polished parabolic searchlight mirror characterized by its persistent retention, when exposed to the action 01' humid air, salt spray, the corrosive products emitted by high-intensity arc electrodes, heat, or mechanical shock, of its parabolic figure and reflectivity, and having substantially the composition: 30% chromium, 5% tungsten, 1% carbon, and the remainder cobalt.

ALBERT V. HARRIS. 

